This application claims priority under 35U.S.C. xc2xa7xc2xa7119 and/or 365 to Japanese Patent Application No. 10-348187 and No. 11-231382 filed in Japan on Dec. 8, 1998 and Aug. 18, 1999, respectively, the entire content of which is herein incorporated by reference.
The present invention relates to a martensitic stainless steel product containing chromium in the range of 9 to 15% by weight, which is mainly used under environments containing hydro-sulfide such as oil wells and gas wells (hereinafter, referred to simply as xe2x80x9coil wellxe2x80x9d) or chemical plants. In particular, the present invention concerns a martensitic stainless steel product which is superior in weatherability under atmospheric environments during transportation and storage, and also superior in corrosion resistance, more specifically, in sulfide stress cracking resistance, even Linder environments containing hydro-sulfide.
With respect to steel products widely used in the application under oil well environments, steel pipes, steel sheets, etc. are listed, and among these, the steel pipes include seamless steel pipes and welded steel pipes.
One of the typical production methods for seamless steel pipes is the so-called Mannesmann-mandrel mill method, and this method is widely used because of its superior dimensional precision and productivity.
Its pipe making process generally consists of a heating process in which a round billet as a material is heated to a predetermined processing temperature, a piercing process in which the heated round billet is formed into a hollow shell by using a piercing mill, an elongating process in which the hollow shell is formed into a pipe for finish rolling by using a mandrel mill, a re-heating process in which the pipe for finish rolling is again heated, and a finish rolling process in which the pipe for finish rolling thus again heated is shaped so as to have a predetermined product dimension by using a stretch reducing mill.
In this case, in general, the heating temperature of the material round billet is set at 1100 to 1300xc2x0 C., the pipe temperature after the elongating process by the mandrel mill is set at 800 to 1000xc2x0 C., the reheating temperature of the pipe for finish rolling is set at 850 to 1100xc2x0 C., and the finish temperature by the stretch reducing mill is set at 800 to 1000xc2x0 C.
In the case of welded steel pipes, a steel sheet as a material is finished so as to have a predetermined product dimension by using a method, such as an ERW (electric-resistance welding)-pipe making method, a UO-(UO press-submerged arc welding)-pipe making method, and a laser welding-pipe making method.
Thereafter, in the case of the steel pipe made of martensitic stainless steel containing chromium in the range of 9 to 15% by weight (hereinafter, referred to simply as xe2x80x9cmartensitic stainless steel pipexe2x80x9d), the product is further subjected to a quenching process at not less than 900xc2x0 C., and then to a tempering process at 600 to 750xc2x0 C. so as to impart a predetermined strength.
During the producing process of such a martensitic stainless seamless steel pipe or steel plate for welded steel pipe, in the case of the seamless steel pipe, it is subjected to a heat treatment of 600 to 1300xc2x0 C. during the respective processes, and in the case of the welded steel pipe, a steel plate is subjected to heating at 600 to 1000xc2x0 C. during a formation process into a steel pipe and a heat treatment process after the pipe formation. For this reason, oxide scales (hereinafter, referred to simply as xe2x80x9cmill scalesxe2x80x9d) inevitably are generated on the inner and outer surfaces of the pipe.
Normally, mill scales are completely removed by a pickling process applied after the shot blasting process. This is because, in general, it is considered that a chromium depression zone exists in the base material steel right under the mill scales and that a desirable corrosion resistance can not be obtained without removing this chromium depression zone as well as the mill scales.
The combination of the shot blasting process and the succeeding pickling process is provided because the application of only the pickling process takes a long time to completely remove the mill scales and the chromium depression zone, resulting in degradation in productivity.
However, the pickling process requires a number of sub-processes and great costs, resulting in degradation in productivity and an increase in the production costs of the products, as well as causing deterioration in working environments due to acid mist, etc. For this reason, from the viewpoints of improvements in productivity, maintenance of good working environments and reduction of the production costs of the products, there have been ever-increasing demands for the simplification of the pickling process, and further, the elimination of the pickling process.
With respect to the shot blasting process, methods are proposed in which grains made of 13% chromium steel, which is the same as the processed steel, or alumina are used as the grains for shot blasting. The reason for this is described as follows: if iron grains are used for the blasting process for stainless steel, pulverized fine particles resulting from the iron grains for shot blasting remain on the surface of the stainless steel product, and in the case when the pickling process is omitted, rust develops from the fine particles of the iron grains for shot blasting serving as starting points in atmospheric environments; this causes so-called rust deposition, resulting in deterioration in the appearance of the products. Moreover, the rust deposition serves as a starting point of the occurrence of pitting corrosion, and accelerates corrosion under actual service environments, such as, high-temperature, high-moisture environments including carbon dioxide gas and hydro-sulfide in the case of oil country tubuler goods.
However, even in the case when the grains for shot blasting made of 13% chromium steel or alumina are used, the martensitic stainless steel containing chromium in the range of 9 to 15% by weight is sometimes subjected to slight corrosion when left in atmospheric environments, if the pickling process is omitted.
Conventionally, there is hardly any researches made on the relationship between the operation conditions of the shot blasting process and the generation of rust. At present, in actual operations, a pickling process for a short period of time is further carried out after the shot blasting process, or the processing time of the shot blasting is extended sufficiently longer than is necessary so as to completely blast and remove the chromium depression zone; consequently, the efficiency of the shot blasting process deteriorates.
However, some researches have been made on not only these grains for shot blasting, but also the shot blasting method itself. More specifically, in a commonly-used shot blasting method which is a so-called pressure blast system, grains for shot blasting are discharged and blasted onto target materials together with compressed air. However, the pressure blast system has the following problems: Running costs increase because of a high power consumption of the compressor, the compressor generates a high pressure, resulting in the possibility of rupturing, and fine grains of shot blasting scatter around, causing degradation in the working environments.
For this reason, a so-called vacuum suction blast system which utilizes the air suction function of an air suction device, has been proposed as a new shot blasting method for shot blasting a pipe inner surface. For example, this method is proposed by Japan Laid-Open Patent Application No. 60-263671. Moreover, blasting devices of the vacuum suction blast system, which enhance the blasting efficiency of this method by adjusting the difference in static pressures or circulating the air flow, have been proposed by, for example, Japanese Laid-Open Patent Application No. 63-22271 and Japanese Laid-Open Patent Application No. 6-270065.
However, the objective of these conventional proposals is to make the vacuum suction blast process more efficient, and it is necessary to apply a pickling process after the shot blasting process so as to completely remove scales.
In recent years, elimination of the pickling process has been demanded as described earlier, and performances of the surface state after the shot blasting process, as it is, have become more important, however, at present, no standard has been established about the extent to which the surface state has to be finished by the shot blasting process in order to ensure a desired corrosion resistance. An excessive shot blasting process causes a reduction in productivity, and an insufficient shot blasting process causes degradation in corrosion resistance.
The objective of the present invention is to provide a martensitic stainless steel product which is superior in rust forming resistance Linder atmospheric environments even when left in a surface state after a shot blasting process as it is, and which is also superior in corrosion resistance, more specifically, in sulfide stress cracking resistance, even under service environments containing hydro-sulfide. The martensitic stainless steel product of the present invention having the surface state after a shot blasting process, as it is, does not require a pickling process during its production; therefore, this product makes it possible to improve the working environments and productivity, and also to reduce production costs.
The steel product of the present invention is a martensitic stainless steel having a chromium content of 9 to 15% by weight, and a surface state such that mill scales generated during the production have been removed from its surface by the shot blasting method. The surface state satisfies the following conditions: when a color image of the surface is analyzed with respect to blue and a tone is obtained, in a histogram of the values of the tone X and the number of pixel Y, the maximum frequency Yp of the pixels and the tone value Xp at which the maximum frequency Yp has been counted have a relationship which satisfies the following inequality:
800Xpxe2x88x92Ypxe2x88x9227000 greater than 0. Here, the number of the pixels of the color image is 640xc3x97480, and the tone values represent values obtained by dividing the tone of the pixels into 0 to 255 classes.
Preferably, the above-mentioned color image is a pickup image of the surface of a steel product, taken with an adjusted luminance of 200 1xc3x97 by using a metal halide lamp.
The surface roughness of the steel product of the present invention is preferably set to have a maximum height Ry of not more than 80 xcexcm, and more preferably not more than 50 xcexcm. More specifically, in the case of the vacuum suction blast system used as the shot blasting method, it is preferably set to be not more than 80 xcexcm, and in the case of the pressure blast system, it is preferably set to be not more than 50 xcexcm. Here, the above-mentioned maximum height Ry refers to the maximum height standardized by JIS B 0601 (hereinafter, the same is true).
The base material may be a martensitic stainless steel which contains 9 to 15% by weight of chromium, preferably further contains not more than 0.5% carbon, not more than 1% silicon, not more than 5% manganese, 0 to 8% nickel, 0 to 7% molybdenum, 0 to 0.1% titanium, 0 to 0.1% zirconium, 0 to 0.1% niobium and 0 to 0.1% sol, aluminum.
With respect to the above-mentioned martensitic stainless steel products, in the case of a steel pipe, the surface state of at least the inner surface satisfies the above-mentioned inequality: 800Xpxe2x88x92Ypxe2x88x9227000 greater than 0, and its surface roughness is set to be not more than 80 xcexcm, preferably not more than 50 xcexcm.
The above-mentioned martensitic stainless steel product is superior in weatherability under atmospheric environments during production, transportation and storage in warehouses or yards, and also superior in sulfide stress cracking resistance, under service environments containing hydro-sulfide in oil wells, chemical plants, etc.